Light splitter and optical transmitter configuration with a light splitter

ABSTRACT

Method for manufacturing a selective light splitter and recombining device that divides white light into red, green and blue light and recombines red, green and blue light independently of polarization, provides two coating systems, each reflecting and transmitting color-selectivity. A transparent base body is embedded in or is on the coating systems and at least one of the coating systems has a layer of a material with lower refractive index and at least one of a material with higher refractive index. The lower refractive index is selected so that 1.7≦N LS ≦2.1 and the refractive index of the transparent base body is selected so that 1.52≧N K ≧1. The method also includes determining an emergent face for the red, green and blue light, and tilting one emergent face such that it forms an angle p deviating from zero by more than is caused by fabrication tolerances for which φ≦5°.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional of U.S. application Ser. No. 09/267,475, filed Mar.11, 1999, U.S. Pat. No. 6,288,844.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to light splitters as well as opticaltransmitter configuration with a light splitter.

The present invention is based on problems such as were recognized whenemploying X-cubes and will be explained in the following. But thefindings can, in principle, be transferred to optical light splitterswhich, as will yet be explained, are used in connection with light ofdifferent polarization. Regarding this topic, reference can be made toA. Thelen, “Nonpolarizing interference films inside a glass cube”, Appl.Optics, Vol. 15, No. 12, December 1976.

For example, in DE 40 33 842 a cuboid optical structural componentcomposed of single prisms with dichroic layers is referred to as a“dichroic prism”. WO98/20383 by the sage applicant.

Definitions

The following definitions are used:

Light,

visible light: light with maximum energy in the spectral range 380nm-720 nm

red light: light with maximum energy in the spectral range 580 nm-720nm, in particular in the spectral range 600 nm-680 nm

green light: light with maximum energy in the spectral range 490 nm-605nm, in particular in the spectral range 500 nm-600 nm

blue light: light with maximum energy in the spectral range 380 nm-510nm, in particular in the spectral range 420 nm-500 nm

yellow light: light with maximum energy in the spectral range 475 nm-605nm, in particular at 582±3 nm

white light: light with red, blue and green light components

transparent: negligible absorption in the spectral range 380 nm-720 nm

cube: spatial shape formed by identical rectangles pairwise opposingeach other in parallel

First, the fundamental effect of an X-cube will be explained withreference to FIG. 1. Optic light splitters of this type are employedprimarily in projectors in order to separate white light into red, greenand blue light or to recombine the latter into white light. According toFIG. 1, an X-cube comprises four single prisms 2 a-2 d. The prismscommonly comprise BK7 glass. In cross section they form right-angledisosceles triangles with an angle of 90°. The length of the hypotenuseis, for example, between 5 mm and 50 mm, preferably 20-30 mm. Betweenthe two prism pairs 2 a and 2 b, on the one hand, and 2 d and 2 c, onthe other, a spectrally selectively reflecting and transmitting coatingsystem 5 is embedded, which largely reflects blue light but largelytransmits green and red light.

Between the two prism pairs 2 a and 2 d, on the one hand, and 2 b and 2c, on the other hand, a further spectrally selectively reflecting andtransmitting coating system 7 is embedded which largely reflects redlight, however largely transmits green and blue light.

Consequently, on the X-cube three channels result for red, green andblue light, K_(R), K_(G), K_(B) and one channel K_(R+B+G) for whitelight. On each of the coating systems 5, 7, between the addressed prismpairs, reflection at 45° of the incident light, thus colored light,takes place. The hypotenuse faces of the prisms 2 can be coated with anantireflection coating system.

Such X-cubes are mainly used today in projection apparatus in order torecombine red (R), blue (B), and green (G) light, each of which issupplied via light valves, in particular LCD light valves, operating intransmission to the associated channels K_(R), K_(B), K_(G) in channelK_(R+B+G) into white light. This is indicated in FIG. 1 in dashed lines.Light valves are therein image-forming elements comprising amultiplicity of individually driven pixels. The number of pixels thereinyields the resolution according to EVGA, SGA, EGA, or XGA standards,etc.

Due to the printed conductor and the driving electronics a lower limitof the pixel size exists in the case of such light valves operating intransmission and it is only with difficulty possible to attain sizesbelow this limit. When decreasing the pixel size, furthermore, theoptical aperture per pixel decreases.

The restriction does not apply in light valves which do not work intransmission but rather, as shown in solid lines using light valves LCDin FIG. 1, operate in reflection and therein rotate the plane ofpolarization of the reflected light by 90°.

The use of such light valves operating in reflection has been hindereduntil today by problems which will be explained later. In FIG. 2 theconditions are shown which obtain when replacing conventional lightvalves, which, according to FIG. 1, are LCD valves operating intransmission, by light valves RLV, which are reflective light valvesoperating in reflection. If, to the configuration according to FIG. 1, alight valve RLV operating in reflection is connected according to FIG.2, for example, reflected S-polarized (direction of oscillation of the Efield) blue light B reflected on coating system 5 of the X-cubeaccording to FIG. 1, is converted on the light valve RLV intoP-polarized blue light and reflected back onto the coating system 5 and,again, reflected by the latter. On one and the same coating system 5,according to FIG. 2, and, analogously, for red light on system 7,reflections of light of identical spectra but different polarizationsoccur.

Spectrally selectively reflecting and transmitting coating systems, suchas are used in said X-cubes but also in other light splitters forcolor-selective effects, are conventionally produced by means ofdielectric multicoating systems. These comprise each at least one layerof a material with lower refractive index and one layer of a materialwith higher refractive index. For example, as the material with lowerrefractive index SiO₂ is conventionally used, with a refractive index of1.46. As the material with higher refractive index, for example, TiO₂ isused today with a refractive index of 2.4 or Ta₂O₅ with a refractiveindex of 2.1.

In FIG. 3 the reflection of S-polarized blue light on a color-selectivecoating system comprising SiO₂/TiO₂ is shown as well as that ofP-polarized blue light on the same coating system. Both measurementstook place at an angle of light incidence of 45°, as depicted in FIG. 2.

In FIG. 4 is shown on a coating system again composed of SiO₂/TiO₂layers and selectively reflecting red light R, the reflection behaviorof S-polarized and of P-polarized red light. The measurements of FIGS.3, 4 were carried out on an X-cube with BK7 glass as the base bodymaterial wherein the listed color-selective coating systems 5, 7 or FIG.2 were embedded.

In FIGS. 3 and 4 is evident that, on the one hand, in both cases thereflection of P-polarized light is significantly less than that ofS-polarized light, quite pronounced on the red-selective coating system,and that further a marked edge shift—polarization shift—of the reflectedspectra takes place. For example, with selective reflection of bluelight the 50% reflection points for S- and P-polarization are spacedover 70 nm apart, corresponding to Δ_(B).

If in FIG. 2 the represented path of rays is considered, without takinginto account the second color-selective coating system 7 provided on theX-cube, thus only the reflection on one coating system, namely thecoating system 5 for blue light, one obtains

I′ _(Bout) ^(P) =I _(in) ^(S) ·R _(RB) ^(P)  (1)

where

I_(Bout) ^(P): intensity of the P-polarized blue light reflected back bycoating system 5,

I_(in) ^(S): intensity of the S-polarized blue light incident on coatingsystem 5,

R_(RB) ^(S): the reflection of the blue-selective coating system 5 forS-polarized blue light,

R_(RB) ^(P): the reflection of the blue-selective coating system 5 forP-polarized blue light.

Starting from the reflection behavior shown in FIGS. 3 and 4 for bluelight B on coating system 5 according to FIG. 2 and, analogously, forred light R on coating system 7, taking into consideration theparticular transmitting coating systems, thus system 7 for blue light Bor system 5 for red light R, respectively, intensity spectra forI_(Bout) ^(P) or for I_(Rout) ^(P) are obtained as shown in FIG. 5 orFIG. 6, respectively. Taking into consideration the statedtransmissions, as well as according to FIG. 2 the reflection behavior ofa potentially added light valve RLV operating in reflection, for bluelight is obtained:

I _(Bout) ^(P) =I _(in) ^(S) ·T _(RR) ^(S) ·R _(RB) ^(S) ·R _(RLVB) ·R_(RB) ^(P) ·T _(RR) ^(P)  (2)

Therein denote further

T_(RR) ^(S): the transmission of the red-selective coating system 7 forS-polarized light

R_(RLVB): the reflection of the light valve

T_(RR) ^(P): the transmission of the red-selective coating system 7 forP-polarized light.

The expression for red light is obtained analogously.

The differing reflection properties of the selective red or blue coatingsystems with respect to S- and P-polarization lead to seriousconsequences:

The light from the spectral ranges Δ_(B) or Δ_(R) between the S- andP-reflection spectra according to FIGS. 3 and 4 is not output but passescorrespondingly the blue or red reflector and remains as scattered lightin the system, according to FIG. 2 in the X-cube.

In FIG. 7 the spectrum of the scattered light in the X-cube is shownwith the stated intermediate spectral ranges Δ_(B) and Δ_(R) accordingto FIGS. 3 and 4 entered.

It is evident that a large quantity of scattered light remains in thesystem. Further, as evident in FIGS. 5 and 6, the total transmission inthe red spectral range as well as also in the blue is insufficient, i.e.substantially less than it would be, in view of FIG. 2, if on bothcolor-selective coating systems 5 and 7 only S-polarized light were tobe reflected.

SUMMARY OF THE INVENTION

Starting from the described problems, the present invention poses thetask of creating light splitters of the above stated type in which thediffering color-selective behavior of the coating systems with respectto differently polarized light, is reduced or corrected, or to create anoptical transmitter configuration with a light splitter with which thisis also attained. As the light splitter of the transmitter configurationaccording to the invention is preferably used in combination a lightsplitter according to the invention.

Further, the scattered light in the light splitter or in the transmitterconfiguration is to be minimized and, if appropriate, the color locationis to be optimized.

This task is solved, on the one hand, in the case of a light splitter ofthe type described in the introduction thereby that for the lowerrefractive index N_(LS) applies:

1.70≦N _(LS)≦2.1.

As will yet be explained, this leads to the fact that the edge shiftΔ—the polarization shift—of the P- and S-reflection spectra, asexplained in conjunction with FIGS. 3 or 4, is substantially reducedand, in addition, the reflection of P-polarized light is approximated tothe value for S-polarized light.

In a preferred embodiment, the refractive index N_(LS) is selected as:

N _(LS)=1.8±2%

As the material with higher refractive index is preferably used amaterial comprising at least as the major component an oxide oroxynitride, therein preferably a material from the series TiO₂, Ta₂O₅,Nb₂O₅, HfO₂, ZrO₂, SiO_(x)N_(y), especially preferred TiO₂ and/or Ta₂O₅.All of these materials have indices of refraction of maximally 2.1.

The lower refractive index N_(LS) selected according to the invention ispreferably controlled thereby that as the associated material a mixedmaterial is used comprising at least two materials m₁ and m₂, for therefractive index N_(m1), N_(m2), to which applies:

N _(m1)≧1.05N _(LS)

N _(m2)≦0.95N _(LS).

The first material m₁ of the mixed material is preferably also amaterial used as the material with the higher refractive index, thus anoxide or oxynitride, therein preferably at least one of the above listedmaterials TiO₂, Ta₂O₅, Nb₂O₅, HfO₂, ZrO₂, SiO_(x)N_(y), especiallypreferred TiO₂ and/or Ta₂O₅.

As the second material m₂ of the mixed material is preferably used SiO₂and/or Al₂O₃ and/or SiO_(x)N_(y) and/or Y₂O₃. By selecting the mixingratio m₁/m₂ of the mixed material, the material with the lowerrefractive index, the desired refractive index N_(LS) is realized. Asthe material with the lower refractive index, a mixed material ispreferably used comprising SiO₂ and TiO₂ with an SiO₂ fractionA_(SiO2)=(60±5)% and a TiO₂ fraction A_(TiO2) of (100%—A_(SiO2)). Y₂O₃,having a refractive index of 1.8±2%, can therein also be used as thematerial with the lower refractive index.

The refractive index N_(K) of the material of the at least one base bodyis preferably selected as follows:

1.52≧N _(K)≧1.

The value 1.52 corresponds therein to the refractive index of BK7 glasswhich is commonly used in particular for X-cubes. The stated body canalso be fabricated of quartz glass with

N _(K)=1.45±2%.

It is preferred if on the light splitter according to the invention bothcoating systems are implemented like said at least one. Furthermore,according to the invention and especially preferred, a light splitterimplemented as an X-cube of the above described type is proposed, inwhich the coating systems are embedded substantially along diagonalplanes of a base body cuboid which is preferably square in a cuttingplane perpendicular to the common line of intersection of the coatingsystems.

To summarize: thus a formulation according to the invention of thesolution of the above task is the specific selection of materials of thedescribed coating systems.

A second solution of the above task is attained through a spectrallyselective light splitter. In such a light splitter, which splits whitelight into red, green and blue light, respectively recombines whitelight from red, green and blue light, with two separate coating systems,each reflecting and transmitting color selectively, which are applied,or are embedded, on or in at least one transparent base body and onwhich the body defines emergent faces for the red, green and,respectively, blue light, at least on of the emergent faces with respectto the direction of the light emerging from it and reflected on at leastone of the coating system is tilted such that the face normal of theemergent face forms with the stated direction, and acute angle φ whichdiffers from 0°. Due to fabrication tolerances on the light splitter,angles between light emergent direction and the face normal differingfrom 0° may well occur in this case, but, on the one hand, suchtolerance-dependent angle deviations are not reproducible and, on theother hand, the tolerance-dependent angle deviations due to productionare prior known. The zero deviation realized according to the invention,in any event, is greater than the stated prior known tolerancedeviation.

Preferably, and as yet to be explained, the addressed maximum angledeviations are 5°.

It is now possible to apply directly onto the emergent faces on such alight splitter light valves, operating in reflection with which, withrespect to the coating systems, for example according to FIG. 2, lightin S-polarization, incident on a considered coating system and to bespectrally divided, is reflected on the coating system at a differentangle than the P-polarized light to be recombined of the same spectrumreflected by light valve RLV.

However, the possibility also exists of realizing the reflection angleshift on one and/or the other coating system without correspondingtilting of the emergent faces on the light splitter itself, i.e. withoutchanging the light splitter itself by mounting the light valves at acorresponding relative tilt.

A corresponding optical transmitter configuration with a light splitter,with an incident and emergent face for white light and emergent faceseach for red, blue and green light, comprises reflectors, operationallyconnected with the emergent faces of the stated light splitter, whichreflectors change the polarization of light and on which the lightemergent at the emergent faces is reflected at an angle deviating from0° by more than is given through the fabrication tolerances of the totalconfiguration. Such an optical transmitter configuration can now berealized through the direct application of light valves, operating inreflection, onto the tilted emergent faces of a light splitter or thereflector tilt can be realized through mechanical mounting measures,apart from the light splitter implementation proper.

The light splitter according to the invention considered by itselfcomprises preferably in combination the measures with respect to layermaterial as well as also the measures of realizing on the coatingsystems differing reflection angles specific to the direction of theincidence of rays, as is specified by the light splitter. Furthermore,an optical transmitter, in principle with tilted reflectors, preferablycomprises also the solution characteristics with respect to specificlayer materials on the light splitter.

The light splitter according to the invention considered by itselfcomprises preferably in combination the measures with respect to layermaterial as well as also the measures of realizing on the coatingsystems differing reflection angles specific to the direction of theincidence of rays, as is specified by the light slitter according toclaim 10. Furthermore, an optical transmitter, in principle with tiltedreflectors according to claim 12, preferably comprises also the solutioncharacteristics with respect to specific layer materials on the lightsplitter as specified according to the wording of claim 13.

Each of the principles according to the invention, namely the materialselection according to the invention of one, preferably of both of thecoating systems and/or reflection specific as to direction of incidenceon at least one, preferably on both coating systems, now permits workingwith reflecting light values in particular also in the case of projectorconfigurations.

An optical transmitter configuration, on which the stated tilt isrealized and/or which comprises a light splitter with the materialselection according to the invention of the coating systems, preferablycomprises further a polarization beam splitter at the input side and/oran HMI lamp as an illumination source.

Said HMI lamp has in its light spectrum low energy values where therange of spectral shift range also is located of the reflection behaviorwith respect to S- and P-polarized light, i.e. corresponding to thespectral ranges Δ_(R) and Δ_(B) according to FIGS. 3 and 7.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic optical transmission configuration of an X-cube,

FIG. 2 illustrates conditions occurring when replacing conventionallight valves,

FIGS. 3, 4, 5, 6 and 7 are each graphs plotting reflectants or intensityof light against wavelength.

The invention will now be explained further by example with reference tothe FIGS.

Therein depict:

FIG. 8 the reflection of blue light on a light splitter according to theinvention with a layer material selection according to the invention forS- and P-polarizations,

FIG. 9 in a representation analogous to FIG. 8 the reflection resultingwith the layer material selection according to the invention of redlight in S- and P-polarization,

FIG. 10 the total transmission of blue light in an X-cube with the layermaterials selected according to the invention, i.e. with coating systemswhich in each instance lead to the results according to FIGS. 8 and 9,as well as the spectrum of an HMI lamp,

FIG. 11 in a representation analogous to FIG. 10 the spectrum of thetotal transmission of red light as well as of said HMI lamp registeredon the same X-cube,

FIG. 12 the spectrum of the scattered light resulting on said X-cubeaccording to the invention as well as of said HMI lamp,

FIG. 13 schematically a first optical transmitter configurationaccording to the invention in the form of a projection configurationaccording to the invention with a light splitter according to theinvention with the materials of the coating systems selected accordingto the invention,

FIG. 14 schematically and only shown with one of the coating systems, anX-cube and light valve configuration in order to explain the inventionunder the aspect of its tilt with respect to red light,

FIG. 15 in a representation analogous to FIG. 14, the conditions forblue light,

FIG. 16 a simple realization possibility of the invention under theaspect of its tilt using a conventionally formed X-cube, and

FIG. 17 spectra of the red and blue light in order to explain the effectof the tilt according to the invention, and

FIG. 18 in analogy to FIG. 12, the residual light spectrum resulting inan X-cube with combined solutions according to the invention accordingto the spectra of FIG. 17, with the spectrum of the HMI lamp PhillipsUHP 120 W.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The results presented in the following in conjunction with FIGS. 8 to12, were measured using a configuration such as is represented inprinciple in FIG. 2. An X-cube was provided with color-selective coatingsystems 5 or 7, respectively. The material with the higher refractiveindex in the coating systems 5, 7 was TiO₂, the material with the lowerrefractive index was a mixed material comprising SiO₂ and TiO₂ in amixing ratio of A_(SiO2)/A_(TiO2)=60/40. The body of the X-cubecomprised quartz glass.

The effects of the transmission, resulting in the reflectionmeasurements on the particular noninvolved coating system—thus for bluelight the red-selective system 7 and for red light the blue-selectivesystem 5—in particular the transmission difference for S and P-polarizedred or blue light, respectively, were negligible.

On the configuration of FIG. 2, in analogy to FIG. 3, in an X-cube withTiO₂ as the material with the higher refractive index and SiO₂ as thematerial with the lower refractive index on the light splitter accordingto the invention, were measured spectra for reflected blue light B in S-and P-polarization, represented in FIG. 8, and analogously for red lightR in FIG. 9. The refractive index N_(LS) within the range

1.7≦N _(LS)≦2.1

was selected as 1.8±2%.

Due to the selection of said mixing ratio of the mixed material withlower refractive index, and thus the setting of the lower refractiveindex, a substantial reduction of the polarization shift Δ_(B) or Δ_(R),respectively, is attained and thus a substantial approximation of thereflection spectra edges of S- and P-polarized light. If, in the case ofprior known optical structural components according to FIG. 3 at 50%reflection a polarization shift of 70 nm results between the reflectionspectrum for S- and P-polarized light, in the case of the structuralcomponent according to the invention a polarization shift of only 30 nmis obtained for this light. Similarly, for red light at 30% reflectionon the structural component according to the invention a polarizationshift of only 25 nm results while the same value on prior knownstructural elements is 50 nm according to FIG. 4.

Further, the maximum reflection of P-polarized blue light according tothe invention and according to FIG. 8, is approximately 92% of that ofS-polarized blue light while in prior known structural componentsaccording to FIG. 3 this value is only 75%. As is readily apparent, withrespect to red light, the maximum reflection attained according to theinvention with P-polarization is 97% of that with S-polarization whilein the case of prior known structural components according to FIG. 4,the former is only 40%.

In FIG. 10 in the case of the system, according to FIG. 2, as definedabove and structured according to the invention the total transmissionof blue light is depicted and, correspondingly in FIG. 11, the totaltransmission of red light, thus over the light division on the X-cube,reflection on RLV and light recombination on the X-cube.

An HMI lamp, for example a lamp UHP 120 W by Phillips, yields the lampspectrum Δ also entered in FIGS. 10 and 11. It is evident that the lampspectrum has low energy or a low intensity where the blue light, due tothe still remaining polarization shift Δ′_(B) according to FIG. 8, has alow transmission in any event. As is extremely advantageous, the lampspectral line is additionally at 580 nm, corresponding to the yellowlines faded out through the coating system provided according to theinvention (see FIG. 10), which prevents a green coloration of the bluecomponent or orange coloration of the red component. Based on thecomparison of the scattered light spectrum of FIG. 12 with thatresulting on a conventional X-cube according to FIG. 7, it is evidentthat the total scattered light losses in the case of the light splitter,realized according to the invention with coating system materialsselected according to the invention, are substantially less than in thecase of a conventional light splitter of said type, in particular alsoif the light splitter according to the invention is combined with an HMIlight source.

In FIG. 13 is depicted the realization of a projector according to theinvention in which, due to the implementation according to the inventionof the optical light splitter structured as an X-cube, namely withcoating system materials selected according to the invention, lightvalves RLV operating in reflection are used.

White light, S-polarized or both, S- and P-polarized, is incident on apolarization beam splitter 13. The coating system 10 of the beamsplitter, in known manner, has the property of deflecting S-polarizedlight by 90° and of allowing P-polarized light to pass. The P-polarizedlight can also be reflected back to the light source by means of amirror (not shown). The S-polarized white light enters the X-cube 12,structured as explained in conjunction with FIG. 2 but comprising thecoating system materials according to the invention. At the threeemergent faces, corresponding to the three color channels, of the X-cube12 are provided light valves RLV^(R), RLV^(G), and RLV^(B), operating inreflection. With a change in the polarization, the light of theparticular associated spectra is there reflected back in P-polarizedform onto the associated color-selective reflection coating systems,implemented according to the invention, and output after recombinationby the polarization beam splitter 13. Thus the light is split in theX-cube in S-polarization into RGB and in the same X-cube recombined fromRGB in P-polarization. The P-polarized light can traverse unhinderedthrough the polarizing beam splitter 13 and is projected via projectionoptics onto a (not shown) screen.

With reference to FIG. 14, the second solution according to theinvention of the above posed task will be explained in principle, thesolution of which, as already mentioned, can be optimally combined withthe just described solution, namely the special coating system materialselection.

Again, the problem to be solved is given thereby that, as has beenexplained in connection with FIGS. 3 and 4, on the particular coatingsystems of a light splitter a decisive polarization shift of thereflected spectra, corresponding to Δ_(B) and Δ_(R), is generated.

In principle, and in view of said Figures, is to be attained:

to shift the spectral edge of the reflected red light in S-polarization(see FIG. 4) toward longer wavelengths;

to shift the spectral edge of the reflected red light in P-polarizationtoward shorter wavelengths;

to shift the spectral edge of the reflected blue light in S-polarizationtoward shorter wavelengths; and

to shift the spectral edge of the reflected blue light in P-polarizationtoward longer wavelengths.

It is highly surprising that all of these conditions can, in principle,be met thereby that the angle of incidence for P- and S-polarized lighton the associated coating systems are selected so as to be different.This approach will be explained in principle with reference, to FIG. 14.Apart from conditions yet to be explained, this Figure showsschematically an X-cube 20 with the one coating system 5 a—the redreflector—as well as a light valve RLV operating in reflection and, forthe sake of clarity, shown only for one channel.

Essential in the solution according to the invention is that the lightincident on RLV arrives on RLV at an angle φ deviating from 0°, withrespect to the face normal F_(RLV) and no longer, as for exampleaccording to FIGS. 2 or 13, at an angle φ=0°.

The requirements made of the spectrum of the red light for a decrease ofthe polarization shift according to FIG. 4 are now met thereby that theangle of reflection α_(R) for the S-polarized red light is selected tobe smaller than the angle β for the P-polarized red light.

Conversely, the conditions with respect to polarization shift of thespectra of the blue light are met thereby that, as shown in FIG. 15, theangle of reflection α_(B) of the blue light in S-polarization on thecoating system 7 a is greater than the angle of reflection β_(B) of theblue light reflected by the associated light valve RLV. According toFIGS. 14, 15 on the red reflector or blue reflector coating system 5 a,7 a obtains

β_(R)=α_(R)+2φ

β_(B)=α_(B)−2φ.

Angle α is controlled through the geometric relative position of thecoating system 5 a according to FIG. 14 with respect to the direction ofthe light incident on channel K_(R+B+G), angle β as a function of thisangle of incidence a as well as the angular orientation of the reflectoron RLV with respect to the light emergent at channel K_(R) or K_(B).

With respect to red light as well as also with respect to blue light, ina preferred embodiment the particular angle α and β is symmetrically45°. If the deviation of the particular angles α, β from 45° is denotedby δ,

45°±δ=α

45°±δ=β

is obtained where the upper sign applies in each case for blue light atreflector 7 a according to FIG. 15, the lower sign for the conditions atthe red channel with reflector 5 a according to FIG. 14. Therefromresults

φ=δ.

In a further preferred embodiment, δ is selected as

0<δ≦5°.

It follows that on the light valve RLV_(R), associated with the redchannel K_(R), and/or on light valve RLV_(B), associated with the bluechannel K_(B), preferable on both, the incident light is to be reflectedat an angle φ greater than 0° and preferably maximally 5°. Since smallangles of incidence φ which deviate from 0° can also be caused byfabrication, this angle φ of 0° can be selected to deviate to an extentwhich is above that caused by fabrication tolerances.

As is readily evident in FIGS. 14 and 15, the particular angle α isdetermined by the relative orientation of the particular coating system7 a or 5 a and the direction of light incidence of the light enteringthrough channel K_(R+B+G) for white light.

The geometric conditions according to the invention on light splittersand valves RLV, in particular X-cubes and light valves RLV, as shownqualitatively in FIGS. 14 and 15, can be realized through specificformation of the X-cube or the light splitter itself, namely by tiltingtheir emergent faces corresponding to K_(R) or K_(B) as well as theircoating systems 5 a and 7 a and direct application of the valves on thetilted faces (in FIGS. 14, 15 in dashed lines at K_(Rφ), K_(Bφ)).Alternatively—and simpler—the “tilting” is realized through thecorresponding positioning of the RLVs and the implementation ofillumination or recombination optics, with the X-cube or light splitterremaining geometrically unchanged.

A simple realization form results according to FIG. 16 and, as readilycomprehensible based on the combined consideration of FIGS. 14 and 15,thereby that an X-cube, such as shown for example in conjunction withFIG. 2, is tilted with respect, for example, to light valves RLVdisposed in parallel.

In FIG. 17 several spectra for red and blue light are shown, as measuredon an X-cube on which both solutions according to the invention werecombined, namely, on the one hand, material selection of the coatingsystems according to the invention, and, on the other hand, layout ofthe angle of incidence according to the invention. These spectra weremeasured on X-cubes leading already to the above described spectraaccording to FIGS. 8 and 9.

In the red spectra of FIG. 17 denote:

(a) the spectrum of the reflected S-polarized red light at an angle ofincidence α_(R) according to FIG. 14 on the red reflector 5, accordingto the invention, of 45°. This spectrum corresponds to the one spectrumof FIG. 9.

(b) The spectrum of the reflected P-polarized red light reflected on thered reflector 5, according to the invention, at β_(R)=45°.

These spectra, again, show clearly the reduction attained of thepolarization shift according to FIG. 9 according to the invention,through the selection of the material of the coating systems.

(c): The spectrum of S-polarized red light on the same red reflector 5according to the invention, reflected at α_(R)=42°.

(d): The spectrum of P-polarized red light reflected on the same redreflector 5 at β_(R)=48°.

Comparison of the spectra (c) and (d) shows readily that the remainingpolarization shift Δ″_(R) is again reduced by practically 50% throughthe specific polarization-dependent layout of the angles of incidenceα_(R), β_(R).

In the blue spectra B of FIG. 17 denote:

(e): The spectrum of reflected S-polarized blue light reflected on theblue reflector 7, laid out according to the invention with respect tothe layer materials, at an angle of incidence α_(B)=45°.

(f): The spectrum of the reflected P-polarized blue light on the bluereflector 7, according to the invention, with β_(B)=45°.

This spectrum corresponds to that of FIG. 8.

Between spectra (e) and (f) the polarization shift Δ′_(B) according toFIG. 8 is evident.

(g): The spectrum of the S-polarized blue light reflected on the bluereflector 7 implemented according to the invention, with α_(B)=48°.

(h): The spectrum of the P-polarized blue light reflected on the bluereflector 7 implemented according to the invention, with β_(B)=42°.

Again, spectra (g) and (h) reveal the further reduction of thepolarization shift with respect to that attained already through thelayer material selection alone according to the invention according toFIG. 8.

In FIG. 18, in analogy to the representation according to FIG. 12, onthe one hand, the spectrum of an HMI lamp, namely the UHP 120 W byPhilips, is shown, on the other hand, the spectrum of scattered light,attained with an X-cube with the coating systems laid out according tothe invention as well as with the polarization-specific layout of theangles of incidence α, β according to the spectra of FIG. 17.

It is readily possible to implement on the light splitter according tothe invention, in particular an X-cube, for example only the coatingsystem 5 forming the red reflector with the materials according to theinvention and to realize the layout of the angles of incidence accordingto the invention, for example according to FIG. 16, by tilting theX-cube.

What is claimed is:
 1. A method for manufacturing an X-cube having aspectrally selective light splitter and recombining device, whichdivides white light into red, green and blue light being output andrecombines red, green and blue light being input substantiallyindependently of light polarization, comprising the steps of: providingtwo coating systems, each reflecting and transmitting color-selectively;embedding said coating systems in at least one transparent base body;providing in at least one of said coating systems at least one layer ofa material with lower refractive index as well as at least one layer ofa material with higher refractive index; selecting for the lowerrefractive index N_(LS): 1.7≦N _(LS)≦2.1; and selecting for therefractive index N_(K) of the at least one transparent base body: 1.52≧N_(K)≧1.
 2. A method according to claim 1, wherein N_(LS)=1.8±2%.
 3. Amethod according to claim 1, wherein the material with the lowerrefractive index is a mixed material comprising at least two materialsm₁, m₂, Wherein: N_(m1)≧1.05 N_(LS) and N_(m2)≦0.95 N_(LS).
 4. A methodaccording to claim 1, wherein the material with the higher refractiveindex substantially comprises an oxide or oxynitride selected from thegroup consisting of: TiO₂, Ta₂O₅, Nb₂O₅, HfO₂, ZrO₂, SiO_(x)N_(y).
 5. Amethod according to claim 1, wherein the material with the lowerrefractive index comprises TiO₂ and/or Ta₂O₅ as well as at least one of:Y₂O₃, SiO₂, Al₂O₃, SiO_(x)N_(y); and the material with the lowerrefractive index comprises at least predominantly SiO₂ and TiO₂, with aratio of the fractions of A_(SiO2)=(60±5)% and A_(TiO2)=(100%-A_(SiO2)).6. A method according to claim 1, wherein the material with the lowerrefractive index comprises Y₂O₃.
 7. A method according to claim 1,wherein, for the refractive index N_(k) of the material of the at leastone base body: N_(k)=1.45±2%.
 8. A method according to claim 1, whereinboth coating systems are implemented like said at least one coatingsystem.
 9. A method according to claim 1, wherein the coating systemsare substantially embedded along diagonal planes of a base body cuboid,which is at least approximately square considered in a cutting planeperpendicular to a common line of intersection of the coating systems.10. A method according to claim 1, wherein at least one of the emergentfaces for the light reflected on at least one of the coating system andresulting from the splitting, is tilted with respect to the direction ofemergence of the light such that its face normal forms with saiddirection an angle p deviating from zero and specifically by more thanis caused by fabrication tolerances of the light splitter, for whichapplies: φ≦5°.
 11. A method for manufacturing a spectrally selectivelight splitter and recombining device, which divides white light intored, green and blue light being output at respective output surfaces andrecombines red, green and blue light input to respective ones of saidoutput surfaces, comprising the steps of: providing at least twoseparate coating systems, each reflecting or transmittingcolor-selectively; embedding or applying said coating systems on or inat least one transparent base body; determining by said at least onebody an emergent face each for the red, green and blue light; tilting atleast one emergent face with respect to the direction of the lightemergent from it and reflected on at least one of the coating systems,such that its face normal forms with this direction an angle p deviatingfrom zero, specifically by more than is caused by fabrication tolerancesof the light splitter, for which φ≦5°.
 12. A method according to claim11, wherein at least one of the coating systems comprises at least onelayer of a material with lower refractive index as well as at least onelayer of a material with higher refractive index, wherein for the lowerrefractive index N_(LS): 1.7≦N_(LS)≦2.1; and wherein, for a refractiveindex N_(k) of the at least one transparent base body: 1.52≧N_(K)≧1. 13.A method according to claim 12, wherein N_(LS)=1.8±2%.
 14. A methodaccording to claim 12, wherein the material with the lower refractiveindex is a mixed material comprising at least two materials m₁, m₂,wherein: N _(m1)≧1.05N _(LS) and N_(m2)≦0.95N _(LS).
 15. A methodaccording to claim 12, wherein the material with the higher refractiveindex substantially comprises at least an oxide or oxynitride.
 16. Amethod according to claim 12, wherein the material with the lowerrefractive index comprises at least one of the following materials:TiO₂, Ta₂O₅, Nb₂O₅, HfO₂, ZrO₂, SiO_(x)N_(y), and the material with thelower refractive index comprises predominantly SiO₂ and TiO₂, with aratio of the fractions of A_(SiO2)=(60±5)% and A_(TiO2)=(100%-A_(SiO2)).17. A method according to claim 12, wherein the material with the lowerrefractive index comprises Y₂O₃.
 18. A method according to claim 12,wherein the refractive index N_(k) of the material of the at least onebase body is: N_(k)=1.45±2%.
 19. A method according to claim 12, whereinboth coating systems are implemented like said at least one coatingsystem.
 20. A method according to claim 12, wherein the device is anX-cube in which the coating systems are substantially embedded alongdiagonal planes of a base body cuboid, which is at least approximately,square considered in a cutting plane perpendicular to the common line ofintersection of the coating systems.
 21. A method according to claim 12,wherein at least one of the emergent faces for the light reflected on atleast one of the coating systems and resulting from the splitting, isdisposed tilted with respect to the direction of emergence of this lightsuch that its face normal forms with said direction an angle p deviatingfrom zero and specifically by more than is caused by fabricationtolerances of the light splitter, for which: φ≦5°.
 22. A methodaccording to claim 11, wherein the device is implemented as an X-cube.23. A method according to claim 11, including, adjacent said outputsurfaces, providing reflectors for respective of said output light,which reflectors change polarity of light as reflected and wherein lightoutput from respective ones of said output surfaces is reflected at anadjacent surface of respective ones of said reflectors under an anglewhich deviates more from 0° than caused by manufacturing tolerances ofthe device, and wherein each of the reflectors is formed byconfigurations of reflecting light valves.
 24. A method according toclaim 23, wherein the light is reflected on the reflectors at an angle φwherein: 0°<φ≦5°, with the angle deviating from zero by more than iscaused by fabrication tolerances.
 25. A method according to claim 23,wherein the light reflection at the specified angle takes place on morethan one of the reflectors.
 26. A method according to claim 23, whereinan incident face of the light splitter for the light to be dividedcomprises one of a polarization beam splitter and an HMI lamp as theillumination source for the incident face.
 27. A method according toclaim 11, including providing at least two coating systems, eachreflecting and transmitting color selectively, said coating system beingapplied or embedded on or in at least one transparent base body, andwherein at least one of the coating systems comprises at least one layerof a material with lower refractive index as well as at least one layerof a material with higher refractive index, wherein for the lowerrefractive index N_(LS): 1.7≦N_(LS)≦2.1; and wherein, for a refractiveindex N_(k) of the at least one transparent base body: 1.52≧N_(K)≧1. 28.A method according to claim 27, wherein N_(LS)=1.8±2%.
 29. A methodaccording to claim 27, wherein the material with the lower refractiveindex is a mixed material comprising at least two materials m₁, M₂, towhich materials: N _(m1)≧1.05N _(LS) and N _(m2)≦0.95N _(LS).
 30. Amethod according to claim 27, wherein the material with the higherrefractive index substantially comprises at least an oxide oroxynitride.
 31. A method according to claim 27, wherein the materialwith the lower refractive index comprises at least one of the followingmaterials: TiO₂, Ta₂O₅, Nb₂O₅, HfO₂, ZrO₂, SiO_(x)N_(y), and thematerial with the lower refractive index comprises predominantly SiO₂and TiO₂, with a ratio of the fractions of A_(SiO2)=(60±5)% and A_(TiO2)=(100%-A_(SiO2)).
 32. A method according to claim 27, wherein thematerial with the lower refractive index comprises Y₂O₃.
 33. A methodaccording to claim 27, wherein the refractive index N_(k) of thematerial of the at least one base body is: N_(k)=1.45±2%.
 34. A methodaccording to claim 27, wherein both coating systems are implemented likesaid at least one coating system.
 35. A method according to claim 27,wherein the device is implemented as an X-cube in which the coatingsystems are substantially embedded along diagonal planes of a base bodycuboid, which is at least approximately, square considered in a cuttingplane perpendicular to the common line of intersection of the coatingsystems.
 36. A method according to claim 11, including at least oneemergent face for the light reflected on at least one of the coatingsystems and resulting from the splitting, is disposed tilted withrespect to the direction of emergence of this light such that its facenormal forms with said direction an angle φ deviating from zero andspecifically by more than is caused by fabrication tolerances of thelight splitter, for which: φ≦5°.
 37. A method according to claim 11,wherein the device is implemented as an X-cube.
 38. A method accordingto claim 11, including, adjacent said output surfaces, reflectors forrespective of said output light, which reflectors change polarity ofreflected light and wherein light output from respective ones of saidoutput surfaces is reflected at an adjacent surface of respective onesof said reflectors under an angle which deviates more from 0° thancaused by manufacturing tolerances of the device, and wherein each ofthe reflectors is formed by configurations of reflecting light valves.39. A method according to claim 38, wherein the light is reflected onthe reflectors at an angle φ wherein: 0°<φ≦5°, with the angle deviatingfrom zero by more than is caused by fabrication tolerances.
 40. A methodaccording to claim 38, wherein the light reflection at the specifiedangle takes place on more than one of the reflectors.
 41. A methodaccording to claim 38, wherein an incident face of the light splitterfor the light to be divided comprises one of a polarization beamsplitter and an HMI lamp as the illumination source for the incidentface.